Process for producing zinc oxide



May 19, 1936. E. FEUER ET. AL

PROCESS FOR PRODUCING ZINC OXIDE Filed Oct. 10, 1931 E anud Fuer Po.u\ Ker 'INVENTORS ATTORNEY Patented May 19, 1936 UNITED STATES PATENT OFFICE PROCESS FOR PRODUCING ZINC OXIDE Emanuel Feuer, Perchtoldsdorf,

Liesin Austria, assignors and Paul Kemp, to smelting Metallurgische und Metallwerlie Aktiengesells cliaft," Pestszenterzsebet, Hungary, a corporation of Hungary Application October 10 1931, Serial No. 568,104

In Germany October 13, 1930 5 Claims. Our invention relates to a process of and appafavorably with the direct process which in a single operation reduces the zinc from ores and immediately burns the zinc, because the product of the indirect process presents a number of properties adapting it for use as a coloring. material and vulcanization accelerator greatly excelling the like properties of the product resulting from the direct process. Nevertheless, in order that the indirect process may compete in practice with the direct process, the former must beoperated at low cost so that the advantage which is gained in the direct process by starting with cheap raw materials is balanced, and consequently, many attempts at improving the indirect process have been made.

Until recently, the zinc was exclusively distilled out of mufiles and owing to the wear of the muflles and the intermittent character of the operation, as well as the high consumption of fuel, the cost of operation was great. Although experiments with directly heated furnaces were made, the quality of the product was unsatisfactory and these attempts "were discontinued. It was pro-.

: posed to pass a masut flame (a residual distillation product of petroleum) over the zinc bath and agitate the latter, for the purpose of securing improved results,.but it was found that the resulting product had a grayv tinge insteadof being white and could only be made white by subsequent thorough roasting. Such subsequent roasting, however, very materially reduces the utility of the product in many respects.

Certain other processes have been proposed in which the main body of the zinc is treated in the furnace either by blowing air through the zinc after the manner of the converter process or by conducting combustion gasesover the zinc. While these processes may have certain. heat economy advantages over others, it has proven impossible. to equal qualitatively the muflle product because a light yellow tinge of the zinc oxide cannot be avoided. The reason for this may be explained as follows:It is well known that when ZnO is heated it is colored yellow but again becomes White upon cooling. At temperatures up to about 800 C. the duration of the heating is not of material importance but if ZnO is heated tovery high temperatures its yellow color cannot be made to disappear even after cooling, and this fact was not heretofore recognized in the art of zinc oxide production.

In consequence of the strongly positive heat tone of the reaction Zn+O=ZnO, or 5 the resulting ZnO is at first exposed to an extraordinarily high temperature which, in the muffle process, is however, almost immediately lowered by the presenceof oxidizing gasesin excess. It is otherwise inthe case of directly heated furnaces, because here the ZnO formed at the beginning of the furnace ,flows over the entire metal bath surface, along which further combustion constantly occurs, so that the high temperature-dependent on the construction of the iurnace-is maintained for a longer or shorter period before cooling. can occur. Even with the use of highly efficient suction devices, it may only require the fraction of a second under certain circumstances to impart a permanent yellow tinge to the material.

In this process, moreover, the saving in fuel is more than ofiset by the materially poorer yield, because in all of, these cases substantial incineration, due to:direct' action of oxidizing gases on the molten zinc, cannot be avoided. This factor re sults in the necessity of burning the zinc vapors only after they no longer have any connection with the zinc bath, as is in fact done in the case of the muffle process. But to do this involves a difiiculty which had' heretofore not been satisfactorily overcome and by reason of which the process operating with direct heating of the furnace was not comparable in all respects with the mufile process-;

By means of our invention, the difiiculty above referred tois completely obviated and a zinc oxide fully equivalent in all respects to that obtained by the mufile process is secured in a directlyheated furnace, by effecting vaporization of the zinc by the direct action on thezinc melt of gases" or vapors (passed over or through the melt) which containno constituents that react with or oxidize the zinc. For the purpose of vaporizing the zinc, we preferably use heating gases from any suitable source, such as from a generator, grate fire, oil fireand the like. Before the gases come intocontact with the zinc bath they are purified in such; manner that any constituents thereof, notably O2 and CO2; which might modify or oxidize the; zinc, are removed. The gases may be" thus purified by passing them through areducing substance, as for example a bed of incandescent coke. Such purified gases may not only be passed over the surface of the bath, but may be passed through the bath, whereby maximum heat efficiency is obtained without danger of incineration, which is unavoidable with normal heating gases.

In the practice of the process, the vaporization of the zinc melt is effected in one chamber and the combustion of the evolved vapors takes place in a second chamber into which the gases, for example air, necessary for combustion are introduced. If coke firing be used, the same may simultaneously serve as a filter for the heating gases, the raw material being placed on the bed of incandescent coke and vaporized thereon. In lieu of the incandescent bed of coke through which the heating gases are passed, the zinc bath may be kept covered with coal or coke; the covering layer again reduces the gases before they come into direct contact with the melt.

Inasmuch'as the reduction of the heating gases proceeds endothermically and excessive cooling with consequent unavoidable retardation of zinc vaporization'may occur unless the operation is properly carried out, it is advisable to operate in such manner that a preponderance of CO is produced in the gas producer. The removal of the slight residues of CO2 and 02 then, has

but a very minor adverse efiect on the temperature of the heating gases.

Production costs may be reduced by increasing the quantities treated. Inasmuch as the speed of vaporization is in direct ratio to the area of the metallic surface whereby the output increases proportionately to an increase in the area of the free metallic surface, the furnace efliciency may be increased by suitably moving or agitating the zinc bath, the movement being either intermittent or continuous. Rotary furnaces have proven very advantageous for this purpose.

The process of our invention also involves a material saving of fuel in that the heat liberated in thecombustion of the zinc vapors is used for melting the zinc, this being accomplished by placing the zinc melting apparatus directly on the combustion chamber, whereby excellent heat transfer is achieved. This arrangement obviates all danger that, by direct melting of the zinc in the oxidizing atmosphere of the combustion chamber, incineration or contamination of the formed zinc oxide occurs.

Another advantage of the process lies in the fact that the secondary ingredients of the treated zinclferous metals, which otherwise would also be oxidized and be contained in the zinc ash and thus rendered completely valueless, remain in metallic form as valuable by-products in the furnace and may be thence removed for further use.

We have shown two forms of the apparatus of our invention in the accompanying drawing, in which, Figure 1 is a vertical sectional view of one form, and Figure 2 is a similar view of the second form.

Referring to Figure 1, I denotes a stack containing a grate 2 adapted to receive a charge of coke 3. The upper end of' the stack is provided with air passages 4, the area of which may be varied by a damper or valve 5. The stack 1 com-. municates by means of a lateral opening 6 with the zinc bath 1 in the vaporization chamber 8 contained in the rotary drum 10 mounted on rollers 9. The rollers may be driven by worm gearing II. The end or the drum adjacent the stack is provided with a chamber l2 filled or substantially filled, with a charge of coke.

A housing I4, containing the combustion chamber I3, is provided with air nozzles l5 and a channel or passage l6, which leads to a suction device (not shown) designed to draw air in the direction of the arrows through the openings 4, the bed of coke in the chamber l2, the vaporization chamber 8 and combustion chamber l3.

H is a hopper through which the raw material is charged; the material descending through the slightly inclined chute l8 into the bath 1. The gases of combustion from the source thereof are reduced by the coke bed in the chamber l2 and effect vaporization of the bath 1 which is agitated by movement of the drum ID. The zinc vapors flow into the combustion chamber l3 and are there burned with the coaction of the air drawn in at 15. The air and gas mixture flows through the channel I6 to an ordinary condensing plant (not shown) in which the resulting zinc oxide separates. The heat of reaction evolved in the combustion chamber [3 heats the chute I8 so that the raw material melts therein.

In the modified form of the apparatus shown in Figure 2, the coke only fills the lower part of the stack I which is closed above. The charging chute I8 terminates above the coke bed 3, so that the zinc which melts in the chute l8 drops on the coke and is there vaporized. It will thus be apparent that the upper part of the stack forms, at the same time, the vaporization chamber 8 from which the zinc vapors pass directly into the combustion chamber I3. In this modified form of the apparatus, the reduction of the heating gases is efiected by the coke charge itself, in that the air is drawn through the bottom air passages 4 and the coke bed 3'. In other respects, the operation is like that of the form of Figure 1.

We claim:

1. The process for producing zinc oxide which comprises charging metallic zinc into a vaporization chamber containing an atmosphere of hot gases substantially free of all constituents capable of reacting with or oxidizing the metallic zinc whereby the latter is vaporized, and effecting combustion of the evolved zinc vapors in a chamber contiguous to the said vaporization chamber.

2. The process for producing zinc oxide which comprises charging metallic zinc through a conduit in a combustion chamber into a contiguous vaporization chamber containing an atmosphere of hot gases substantially free of all constituents capable of reacting with the metallic zinc whereby the latteris vaporized, and passing the evolved vapors, through and efiecting combustion thereof in said combustion chamber.

3. The process for producing zinc oxide which comprises passing hot gases through reducing material to substantially free said gases of all constituents capable of reacting with metallic zinc, introducing said hot reduced gases into a vaporization chamber, charging metallic zinc into said chamber into direct contact with said hot reduced gases to evolve zinc vapors and. passing said vapors through and effecting combustion thereof in a combustion chamber contiguous to said vaporization chamber.

4. The process for producing zinc oxide which comprises passing hot gases through reducing material to substantially free said gases of all constituents capable of reacting with metallic zinc, introducing said hot reduced gases into a vaporizing chamber, passing a charge of metallic zinc through a conduit in a combustion chamber heated by the burning gases from the vaporizing chamber into direct contact with the hot reduced gases in said vaporizing chamber, to evolve zinc vapors from said charge, passing said zinc vapors t into and through said combustion chamber and introducing air into said combustion chamber to effect combustion of said vapors.

5. The process for producing zinc oxide which comprises continuously introducing into a chamber hot gases or vapors substantially free of all constituents capable of reacting with metallic zinc, continuously introducing an oxidizing atmosphere into a chamber contiguous to the first mentioned chamber, continuously introducing metallic zinc into the first mentioned chamber into direct contact with the hot gases to vaporize said metallic zinc and continuously drawing ofi the vapors from said first mentioned chamber through said second mentioned chamber to oxidize said vapors.

EMANUEL FEUER.

PAUL KEMP. 

